Accumulated electrostatic charges on the high sensitive electronic components (e.g. computer integrated circuits, magnetic tape or disk and etc) are facing the difficulty of draining off to ground when they are placed on the surface of a good electrical insulator (such as wood, rubber and plastic). On the other hand, if these components are in contact with a good electrical conductor (i.e. metallic material), electrostatic will be discharged from them through the conductor in a very high rate and subsequently causes the unwanted current sparks. Both of these phenomenons can cause permanent damage to all these sensitive and expensive electronic components.
People manufacturing the sensitive electronic components are the most significant source of static electricity. As the result, it is essential to discharge static electricity from people who may come in contact with these sensitive electronic components. Working areas where which such sensitive electronic components are manufactured must have a grounded floor or grounded mats which are able to discharge the electrostatic from people walking on the floor or mats by conducting the static electricity from the people to the ground. The only contact people have with the floor is usually through their shoe soles. The conventional shoe soles (made from various types of electrical insulator) do not readily discharge static electricity from the body of the person to the floor. Therefore, it is necessary to have a pair of antistatic or electrically conductive shoes (which include their antistatic or electrically conductive soles) that can effectively discharge the accumulated static electricity from the person body (by creating an electrical conduit).
Technology of antistatic and electrically conductive footwear fabrication is well-established and it can be traced back to year 1941 [U.S. Pat. No. 2,261,072]. Majority of the antistatic, electrostatic dissipative and electrically conductive footwear available in the current market are produced with soles made from synthetic polymers or blends of synthetic and natural polymers.
U.S. Pat. Nos. 4,861,805 and 5,426,870 both disclosed antistatic shoe outer-soles could be made from polyurethane. Antistatic shoes with inner-soles and outer-soles made from vinyl chloride copolymer composition were reported respectively by U.S. Pat. Nos. 5,319,867 and 4,127,552. U.S. Pat. No. 5,426,870 also described antistatic shoe inner-sole that was made by using blend of natural rubber (non-chemically modified type) and styrene butadiene copolymer. Antistatic shoe soles (both outer and inner types) made from synthetic polymers were claimed to be lighter in weight, longer lasting and non-marking on floors if compared to natural rubber based shoe soles. However, it is also known that shoe soles made from all these synthetic polymers have shown some disadvantages, e.g. poor and inhomogeneous fillers dispersion for polyurethane based compositions, quick ageing problem for polyester based polyurethane due to hydrolysis (reaction with water) and also hard thermoplastic materials that reduce shoes wearers comfort.
All these synthetic polymers (i.e. polyurethane, vinyl chloride copolymer and styrene butadiene copolymer) used to manufacture antistatic and electrically conductive shoe soles are categorised as good electrical insulator in their virgin form. They could be turned into electrical conductors by addition of several types of conductive filler. The most common used electrically conductive fillers for antistatic, electrostatic dissipative and electrically conductive shoe soles manufacturing are carbon blacks [U.S. Pat. Nos. 5,233,769, 5,426,870, 5,319,867 and 7,424,782 B2].